1. Technical Field
The RF spectrum is increasingly dense and complex, with expanding introduction of commercial, civil and military emissions, which places extreme demands on the reconnaissance, surveillance and early-warning equipment and missions. The modern weapons and delivery systems have also evolved to be increasingly silent and stealthy, with supersonic/hypersonic delivery speeds, placing naval platforms at high risk. As a result, the need for simultaneous wide-instantaneous-bandwidth, high sensitivity and precision direction-finding is paramount to military success.
Similarly, reconnaissance and surveillance activities need instantaneous-bandwidth, high sensitivity and precision direction finding to overcome scan-on-scan-on-scan (antenna on antenna on frequency) issues, for detecting and analyzing signals-of-interest in the modern crowded environments. This invention is a volumetric Radio Frequency (RF) receiving methodology, utilizing the Half Maxwell Fish-eye (HMFE) Lens for high-gain multi-beam contribution, a frequency independent feed structure to insure reception, element combined to form row and column, and processed to provide precision direction-finding of RF signals, emanating from navigation, radars and communication equipment on surface and airborne platforms. Such a system may be utilized for situation awareness, threat detection and warning, and may be used for cueing combat weapon and/or electronic attack systems. Further, such a system may be used on satellite, naval surface platforms, tactical aircraft and/or mobile/fixed land sites.
2. Prior Art
Modern signal detection and direction-finding techniques utilized multiple dispersed antenna/receiver channels to form vertical and horizontal baselines, per quadrant and use differential antenna/receiver phase, amplitude or time for estimating signal direction, or direction finding. As an example, to provide near hemispheric direction finding, the modern day system utilizes multiple-baseline multiple-quadrant linear interferometers, where each linear interferometer only provide a useful field-of-view of typically 90 degrees in either azimuth or elevation. Therefore, two linear-interferometer baselines are required per quadrant, one for azimuth and one for elevation, where each interferometer is composed of multiple phase-matched and/or calibrated element/receiver channels.
As the emitter approaches the collection system, the elevation angle increases and can approach the apex, where all DF information is severely compromised if not lost (commonly referred to as the donut pattern, caused by shortening of the electrical-length of the antenna baseline as a function of COS(θel) where θel is the elevation angle. Further, interferometry requires use of wide field-of-view antenna elements, sufficiently wide for instantaneous quadrant field-of-view, which severely limits antenna gain, which directly impacts system sensitivity and direction finding accuracy; and provides virtually no spatial filtering for signal discrimination and interference mitigation.
The typical two dimensional interferometer system can be composed of 5 antenna/receivers for azimuth, another 5 antenna/receivers for elevation, times 4 quadrants, which can add up to 40 or more antenna/receivers, with each channel requiring complex detection and measurement processing. For these reasons a near hemispheric direction-finding system can be very large, expensive, require substantial complex processing and plagued with maintenance/calibration issues. This disclosure will provide the requisite performance while substantially reducing the required RF channels, size and cost as compared with these prior art systems.
Some available publications describing the prior art includes:    (1) R. L. Goodwin, “Ambiguity-Resistant Three- and Four-Channel Interferometer,” Naval Research Laboratory Report, September 1976. pp all.    (2) Roger D. Oxley, “Ambiguity-High-Resolution, Six-Channel Interferometer Development and Performance,” Naval Research Laboratory Report, September 1976. pp all.    (3) James Bao-Yen Tsui, “Microwave Receivers With Electronic Warfare Applications,” Krieger Publishing Company, Malabar, Fla. pp 1-4, 93-111.